Network converters such as those described above, are fitted with a transformer and a switch (typically MOSFET) that periodically connects a transformer winding to the input source, that is to a network voltage rectified by a diode bridge and filtered by a capacitor.
Converters need a control circuit that determines the turn-on and turn-off times of the MOSFET so as to supply the load with the power required, at a preset and stabilised voltage. These functions are normally mainly incorporated in an integrated circuit together with the others, that guarantee correct functioning of the converter also in the phases of turn-on and turn-off, and to prevent catastrophic breakdowns if the converter is brought to work outside the scheduled functioning conditions.
For all these reasons integrated control circuits are normally fitted with the function normally called Undervoltage Lockout (UVLO).
A typical circuit as that mentioned above is shown schematically in FIG. 1.
The network voltage Vac is applied by activation of the switch SW to a diode bridge 10, and then to a filter capacitor Cf. The voltage Vin, at the terminals of the capacitor Cf, is applied to the start-up circuit 11, which in the simplest case is constituted by a resistance, and supplies a current Is. The current Is loads a capacitor Cs. The voltage coming from a secondary Wa of the power supply transformer is also applied to the capacitor Cs, through a resistance Rr and a diode D. A fraction Iq of the current Is supplies the integrated control circuit 12. It is applied both to the block UVLO 13, and to the drive circuit 14 of the power supply, that supplies the command voltage Vg to the power MOSFET. The block UVLO 13 comprises a comparator 15 with hysteresis that compares its supply voltage Vcc, with a start-up voltage Vss. The output voltage of the comparator 15 commands a commanded switch SW1 that opens or closes the supply of the drive circuit 14. The voltage Vin is the voltage that will be applied to the power switch of the power supply.
The supply network Vac is applied to the power supply by closing the switch SW and the filter capacitor Cf is loaded in very few milliseconds at the peak network voltage, giving origin to the voltage Vin.
The start-up circuit 11 supplies a current Is that partially loads the capacitor Cs, while a part Iq is absorbed by the integrated control circuit 12. The absorption Iq of the latter in these conditions is very small as the circuit UVLO 13 keeps the switch SW1 open. The current supplied by the start-up circuit 11 thus goes mainly to load the capacitor Cs thereby increasing the voltage Vcc at its terminals.
The voltage Vcc continues rising until it reaches the start-up value Vss, in a time that is variable usually from several hundred of milliseconds to a few seconds. In all this time the drive circuit 14 remains off, and its output voltage Vg, driving the gate of the MOSFET, remains at zero. As soon as the voltage Vcc reaches the voltage Vss, the comparator 15 closes the switch SW1, therefore the current Iq increases considerably; the drive circuit of the MOSFET is enabled and the activity of the power supply starts.
The increased consumption of the device is not supported by the start-up circuit 11 so that there is a rapid decrease of Vcc. This is the reason why the comparator of the circuit UVLO 13 has a hysteresis. To turn the drive circuit 14 off again and to return to the conditions that were present before the start the Vcc has to go down below a second threshold Vstop<Vss, called exactly UVLO. If this hysteresis was not present there would be a continual alternation of turn-ons and turn-offs.
In the meantime, by effect of the switching of the MOSFET, the output voltage of the power supply increases rapidly and with it the voltage of the winding Wa, proportional to it, coupled to the transformer driven by the MOSFET. The winding Wa, the resistance Rr, the diode D and the capacitor Cs, constitute the circuit commonly indicated with the name of auto-supply, the task of which is to support the functioning of the integrated circuit at normal operation. The number of turns of the winding Wa is to be chosen suitably so that the voltage generated by it is greater than Vstop, and the capacitor Cs is to be chosen suitably so that the voltage generated by the winding Wa becomes greater than the voltage Vstop before the voltage Vcc becomes less than the voltage Vstop.
The presence of the voltage threshold Vstop also ensures a defined and safe operation during the turning-off phase. In fact, by opening the switch SW the power supply is fed at the expense of the load present in the capacitor Cf, so that its voltage falls rapidly. As soon as this becomes insufficient to keep the power supply active with the load applied at that moment, the output voltage will diminish rapidly and, with it, Vcc, until it falls lower than the voltage Vstop. As soon as this happens the drive circuit 14 is turned off, Iq returns to its very low initial value, Vg goes to zero and the MOSFET turns off.
Ideally, the voltage generated by the winding Wa, present at the terminals of the capacitor Cs, is hooked through the turns ratio of the transformer to the regulated output voltage and therefore it is also kept regulated by the regulating system. In the actual operation this is quite close to being true, upon variation of the input voltage of the power supply, while the situation is very different upon variation of the load.
This is mainly due to the parasitic parameters of the transformer, by effect of which with high load the voltage rises far more than scheduled by effect of the peaks present on the positive fronts of the voltage on Wa, while with low or negligible load, where the peaks are much lower and the load on Wa is represented by the integrated control circuit 12, can also be greater than that in output, the voltage diminishes considerably below the expected value.
In the more modern integrated control circuits 12, this is accentuated by the adoption of several special techniques aimed at minimizing the consumptions of the power supply at low loads so as to facilitate compliance with the most recent regulations regarding the reduction of consumption of equipment in non-operative conditions (for example EnergyStar, Energy2000, Blue Angel, etc.). These techniques basically entail the reduction of the operative frequency of the power supply at minimum or negligible loads; therefore the energy that Wa is capable of transferring is diminished.
Another problem is represented by the fact that the voltage Vcc cannot exceed a determined value Vccmax for questions linked to the technology of the integrated control circuit 12 that impose limits to the voltage applicable to it and, at the same time, in conditions of minimum or negligible load, Vcc has to stay greater than Vstop, otherwise the system will function intermittently. The variations of the voltage generated by Wa are therefore limited, with some margin of safety, within the interval Vstop−Vccmax.
In addition, in short circuit conditions, the peaks generated on Wa are particularly high and can be sufficiently energetic to keep the Vcc above Vstop, where, ideally, the voltage generated by Wa should be close to zero.
To limit the phenomenon of over-high voltage at maximum load and to ensure intermittent operation in short circuit conditions, as well as optimising the constructive methods of the transformer, generally the resistance Rr is used in series with the diode D with the purpose of smoothing the peaks. Sometimes, as an alternative, a small inductor is used. However, both solutions accentuate the decrease of Vcc at minimum or negligible loads. Also optimising the value of this resistor or inductor (that is, using the minimum value) so as to ensure functioning in safe conditions both at maximum load (Vcc<Vccmax) and in short circuit (Vcc<Vstop), it is difficult to fulfil the condition Vcc>Vstop at minimum or negligible load. To resolve this latter problem a ballast load is added to the power supply so as to contrast the decrease of Vcc. This, however, worsens the efficiency of the system and, above all, makes it practically impossible to comply with the various EnergyStar, Energy2000, Blue Angel, etc.
The same also goes for other external circuitry solutions intended for minimizing the effect of the peaks. In all, meeting the conditions Vcc<Vccmax at full load and Vcc<Vstop in short circuit, makes it extremely difficult to also fulfil the condition Vcc>Vstop at minimum or negligible load.
To minimize the effects of the variations of Vcc it is necessary to extend the interval Vstop−Vccmax as far as possible. Nevertheless, if Vccmax is sufficiently high it is not difficult to fulfill the condition at zero load increasing the number of turns of Waux which however, at high load, produces high voltage which, even though tolerable by the integrated circuit, can easily present problems of power dissipation internally (equal to the product Vcc·Iq), without taking into account the fact that a high Vccmax entails the use of costly technology. If Vstop is very low (compatibly with the safety limits for driving the MOSFETs) it will be easier to fulfill the zero load condition, however it will be difficult to fulfill the condition on the short circuit.
To improve the stability of the voltage Vcc, a possible solution is that shown in FIG. 2. The emitter of a PNP type Transistor T is connected to the transformer Wa, its base is connected to ground by means of a resistance R A capacitor C is connected between the base and the emitter of the Transistor T. The collector of the transistor T is connected to the anode of a diode D, whose cathode is connected to the clamping capacitor Cs and then to the integrated control circuit 12.
On the positive fronts of the voltage generated by Waux the capacitor C filters the peaks.
This system effectively stabilizes the Vcc starting from low loads up to full load and ensures that the condition Vcc<Vstop in short circuit for the converter can be easily obtained. At very low or negligible load, however, it cannot keep the Vcc stable, that decreases considerably, worse than in the case of the circuit of FIG. 1. In fact, the transistor T introduces an additional fall of voltage (Vcesat) and, above all, masks partially or completely the horizontal section of the voltage of Waux which is very short. On the contrary to what happens at full load, in these conditions the pulses, even though being small, would give a small addition of energy capable of obstructing, at least partially, the tendency of Vcc to decrease.